Prior work on cryogenic resistive bolometers [Irwin, 1998, Irwin, 1995, Lee, 1996] has shown that greatly improved performance is attainable from bolometers which possess a sharp transition between two phases with widely differing resistivity, specifically the superconducting (zero dc resistance) state and the normal metallic state. With a properly chosen bias circuit and bias point, these superconducting transition-edge bolometers will operate in a regime of strong electrothermal feedback, exhibiting higher bandwidth, lower 1/f resistance noise, and greater immunity from operating temperature fluctuations. The magnitude of these improvements is determined by the sharpness of the resistive transition (which can be precisely quantified, see below).
It would be highly desirable to achieve these performance advantages in bolometers which operated near room temperature, eliminating the need for cryogenic cooling. However, no materials are known which possess a resistive transition near room temperature that is analogous to the superconducting transition. Therefore, current room-temperature bolometers [Wood, 1997] operate on resistivity-temperature characteristics that are quite gentle, typically exhibiting temperature coefficients of resistivity (TCR) characteristic of a normal semiconductor, i.e., 1-4%/K, rather than on a sharp transition. With such low TCR values, the performance advantages due to electrothermal feedback are insignificantly small. Large arrays of these uncooled microbolometers form the core of many IR camera systems.
The cameras which are being produced today offer impressive performance: net equivalent temperature differences (NETD) as low as 24 mK between adjacent pixels and formats up to 320.times.240 pixels translate directly to unprecedented thermal scene resolution at video frame rates. The best of these systems rely on the temperature dependent resistivity of mixed vanadium oxides in a semiconducting amorphous form, operating at room temperature, as the bolometric sensing mechanism. The key technical breakthroughs which have enabled the technology to reach its current state include engineering of free-standing low-thermal-conductivity membranes (onto which the vanadium oxide is deposited to form a bolometric pixel element), the scale up of this technology to high-fill-factor large-format arrays, and the integration of direct CMOS read out electronics underneath each pixel.
It is widely understood that in order to achieve high performance in a microbolometer sensor, it is necessary to use a material with a high TCR, and that the metal-semiconductor phase transition provides very large TCR values--some two orders of magnitude larger than the semiconducting bolometer materials conventionally used (see FIG. 1). The idea of operating an uncooled bolometer on the metal-semiconductor transition has been suggested before in the open literature [Scott, 1976, Wood, 1997, Jerominek, 1996], and mentioned as an alternative embodiment in a prior U.S. patent [Wood, 1995]. However, all measurements of the metal-semiconductor phase transition, whether in vanadium dioxide [Jerominek, 1993] or in other materials [Tsuda, 1991], exhibit some degree of hysteresis in their R(T) characteristic. This hysteresis, and the 1/f noise that frequently accompanies it, destroys all performance advantages associated with the very high TCR. A typical hysteresis loop width in thin film VO.sub.2 is 5.degree. C. Under special growth conditions, hysteresis widths as low as 1.degree. C. have been reported [Kim, 1994 #132] in thin films, while in bulk (not thin film) form, single crystals can exhibit hysteresis widths as low as 0.15.degree. C. [Kim, 1994].
In previously described uncooled bolometers, the bias is applied either as a dc voltage or current [Jerominek, 1996] or as a short pulsed voltage [Wood, 1997]. In the case of a dc bias, the response to a small modulated IR signal is determined by the TCR of a "local" R(T) characteristic (see FIG. 2), i.e., the resistance changes resulting from small temperature excursions. This TCR within a local R(T) characteristic is typically no more than 8%, little better than in the semiconducting state. For the case of a short pulsed bias, the length of the pulse is too short for electrothermal feedback to be effective, and the full 1/f noise associated with the thin film in its transition is observed, again negating any advantage of the high TCR.
A technique called "correlated double sampling"(CDS) is commonly used for the readout of optical charge-coupled detectors (CCD's). In that case, the signal is given by the difference in voltage during a short "reset" period and a longer "data" period, while the present invention involves no such differencing. Furthermore, in the case of CDS for CCD readout, the bias during the data period is constant and the selection of critical bias and readout parameters is based on the detailed physics and design of the CCD, for example well capacity.